1. Field of the Invention
The present invention concerns technical means especially capable of improvement in high-temperature durability and expansion of the turn down ratio (TDR) in catalytic combustion systems which are used mainly in heat sources and heating applications for catalytic combustion of gaseous fuels or liquid fuels.
2. Related Art of the Invention
Various types of catalytic combustion systems, which use a catalyst body exhibiting an oxidation activity against the fuel to cause catalytic reactions to take place at a surface of the catalytic body, have been known in the art, and their typical combustion method is a premix type structure as shown in FIG. 1.
Referring first to FIG. 1, there is shown a commonly-used premix type structure, in which a fuel gas supplied from a fuel supply valve 1 is mixed with air supplied from an air supply valve 2 in a premix chamber 3, and is delivered to a preheat burner 5 through a premix gas inlet port 4. This premix gas is ignited by an ignition unit 6, thereby forming a flame at the preheat burner 5. High-temperature exhaust gases as a result of such flame formation pass through a catalyst body 8 disposed in a combustion chamber 7 while heating the catalyst body 8, and are discharged from an exhaust port 9. When the catalyst body 8 is heated up to reach its catalyst activity temperature, fuel supply is temporarily stopped by the fuel supply valve 1 to put out the flame formed at the preheat burner 5. Thereafter, by an immediate resupply of fuel, catalytic combustion is started again. The catalyst body 8 enters a high-temperature state. Through a glass 10 located upstream of and in a face-to-face arrangement with the catalyst body 8, the catalyst body 8 radiates heat while releasing heat in the form of exhaust gas for application of heat, heating, and drying. In the foregoing premix type structure, premix gases, whose excess air ratio (i.e., the ratio of an actual amount of air to the air amount theoretically required for fuel complete oxidation) is not less than 1, are constantly supplied to the catalyst body 8, in other words, the catalyst body 8 is operated in an atmosphere excessively abounding with oxygen.
In the above-described conventional catalytic combustion system, a high-temperature atmosphere is produced which is accompanied by the constant coexistence of oxygen at a reaction center position of the catalyst body. As a consequence, constituents of a catalyst are inevitably subject to deterioration by heat. Generally, metals of the platinum group, such as Pt, Pd, and Rh, are frequently used as catalysts for combustion in view of their heat resistance and reaction activity. However, the problem of using such metals is that at high temperatures (from 800 to 900 degrees centigrade), it is difficult to attain steady combustion performance for a long time because of reduction of the active spot count due to aggregation and transpiration of precious metal particles. In premix-type catalytic combustion systems, owing to the drop in activity, the reaction center position is shifted toward the downstream side of the catalyst body, therefore resulting in failing to maintain complete combustion. In addition to such a drawback, in the system making utilization of radiation heat from a catalyst upstream-side surface, the quantity of radiation heat decreases as the service time increases.